In a wireless communications system, a power amplifier is typically used in order to amplify a modulated communications signal prior to transmitting the signal through an antenna. When the power amplifier is placed within a portable wireless communications device, a group of solid-state amplifier devices can be used in order to provide the amplified signal. In this type of communications device, the output of each solid-state amplifier element is combined through the use of a high frequency power combining structure which provides a single output to the communications antenna. Thus, when designing a high frequency power amplifier combining structure, a premium is placed on the capability of the structure to combine signals with minimal loss.
In a conventional power combiner, such as a Wilkinson power combiner, a quarter wavelength section of transmission line is coupled to an output of each solid-state amplifier. At the opposite end of each quarter wavelength section, each transmission line is coupled to one or more similar quarter wavelength sections of transmission line in order to form a single output. In a multi-stage Wilkinson power divider, the combined output of two or more transmission lines is coupled to a second quarter wavelength section of transmission line. The output of these second lengths of transmission line is then combined into a single transmission line section. This method of combining successive stages continues until the outputs of all of the solid-state amplifier devices have been combined into a single output.
As each stage of a multi-stage Wilkinson power combiner requires a quarter wavelength section of transmission line, the length of the multi-stage power combiner can quickly become significant as the number of stages increases. Additionally, each quarter wavelength section of transmission line introduces a corresponding amount of resistive loss in signal strength. Further, as the multi-stage Wilkinson power combiner must generally transform the output impedance of each solid-state amplifier to a standard impedance value, such as 50 Ohms, high impedance sections of transmission line may be required as dictated by the particular impedance profile used to achieve the match. Typically, these high impedance sections further increase the loss of the multi-stage Wilkinson power combiner.
Thus, it is highly desirable for power combining structures to minimize loss, as well as reduce the required length of the structure. This would allow more efficient combining of power from solid-state amplifier devices as well as provide additional savings in the integrated circuit chip area required to perform the power combining function.